y = x2 / 810, 0 <= x <= 90
y is PPC damage and x is distance in meters.
The raw data I gathered was the number of headshots required to kill a mech at a fixed distance.
Distance - Headshots To Kill
-----------------------------------------
90m - 4
80m - 4
70m - 6
60m - 7
50m - 11
40m - 17
30m - 30
20m - 54
13m - 134
First I calculated the min and max damage per shot and then got the average of those 2 numbers which I graphed over the distance from the target. I assumed a headshot takes 33 damage because that was the number I saw most often in these forums.
What I get when graphing it out:
![Posted Image](http://i.imgur.com/7bdlM74.png)
Based on the shape of the graph I assumed this was a y=x2 type graph and the 810 divisor makes the curve hit 10 damage at 90m and keeps things nice and simple, occams razor and all that jazz. Also as a point of reference you hit about 5 damage per shot around 64m.
Link to spreadsheet with more detailed calculations
To summarize:
damage falls off exponentially not linearly below 90m
half damage at around 64m
at 10m it's about like using a machine gun